Sp1 inhibits proliferation and induces apoptosis in vascular smooth muscle cells by repressing p21WAF1/Cip1 transcription and cyclin D1-Cdk4-p21WAF1/Cip1 complex formation

Atherosclerosis and restenosis are common vascular disorders that involve excess proliferation of smooth muscle cells (SMCs) in the artery wall. In this study we demonstrate the anti-mitogenic, pro-apoptotic role of the zinc finger transcription factor Sp1 in vascular SMCs and define the underlying molecular mechanism via its capacity to repress the expression of the cyclin-dependent kinase inhibitor p21(WAF1/Cip1) at the level of transcription, mRNA, and protein. SMC proliferation inducible by a dominant-negative mutant form of Sp1 was abrogated by antisense strategies targeting p21(WAF1/Cip1). Conversely, antisense p21(WAF1/Cip1) induced apoptosis in SMCs overexpressing dominant-negative-Sp1. p21(WAF1/Cip1) overexpression alone stimulated proliferation and inhibited apoptosis. Sp1 down-regulated p21(WAF1/Cip1) expression in SMCs. Sp1 blocked assembly of cyclin D1-Cdk4-p21(WAF1/Cip1) complex formation whose integrity is critical for G(1)->S transition. Moreover, Rb phosphorylation, which lies immediately downstream of the cyclin D1-Cdk4-p21(WAF1/Cip1) complex, was blocked either by Sp1 overexpression or antisense p21(WAF1/Cip1). These findings, using complementary approaches, demonstrate the inverse relationship between Sp1 and p21(WAF1/Cip1) in SMCs and the capacity of Sp1 to regulate SMC proliferation and apoptosis via its repression of p21(WAF1/Cip1).